Hearing devices, such as hearing aids, provided for aiding hearing impaired people in hearing, comprises one or more microphones. The one or more microphones is configured to receive an audible sound signal, typically a speech signal. The sound signals are picked up by one or more sound inlets of the microphone and are within the microphone transferred to an electric signal. The electric signal is transferred to an amplifier, which amplifies the electric signal information to such a level, at which a hearing impaired is able to hear the sound. The amplified sound is transmitted to a receiver, which transduces the electric signal into an audible signal suitable for human hearing and transmits it to the eardrum of a user.
Different kinds of microphone types exist, and common to all microphone types (such as condenser microphones, e.g. electret and MEMS type microphones) is that such microphone units are sensitive to displacement, movement and vibrations as well as the sound pressure level (SPL) to which they are exposed. Imperfections in the microphone performance may arise, when microphones are exposed to environmental changes within the hearing aid device, such as vibrations caused by the receiver.
One factor causing imperfections of microphones in hearing aids is often due to the arrangement of a receiver in close proximity to a microphone. When a receiver emits amplified sound signals small vibrations easily occurs. Such vibrations are distributed throughout the hearing aid shell and internal parts, and are likely to influence the mechanisms of the microphone. The vibration causes the microphone to create an unwanted electrical signal, which gets amplified and transmitted by the receiver to the ear of a user. The amplified signals due to vibrations are thus unwanted signals which are transmitted to the ear drum of a user and which easily forms part of an acoustical feedback loop causing unwanted and annoying sound signals for a hearing aid user.
In hearing aid applications, the sensitivity of microphones to vibrations is a limiting factor in view of the maximum gain that can be applied in hearing instrument platforms. When applying an insertion gain to compensate for the normal amplification provided by the ear structure of a human, this gain factor may from these microphone imperfections unintentionally enhance unwanted signals not forming part of the audible signal of interest. For avoiding at least some of these microphone imperfections, hearing aid designs carefully take into consideration the mounting and arrangement of the receiver and the microphone in relation to each other.
Accordingly, it is of interest to compensate for the sensitivity of the microphones to vibrations arising e.g. in a hearing aid housing structure. Current solutions, such as disclosed in EP2552128 solves the vibration sensitivity problem by using a microphone construction with two diaphragms, such that three chambers are provided in the microphone construction. The two diaphragms are arranged so as to move in opposite directions when the microphone construction is moving downwards or upwards in view of mechanical vibrations. However, this construction requires a somewhat complex microphone construction for the vibrations to be cancelled out.
Accordingly, there is a need to provide a solution that addresses at least some of the above-mentioned problems. At least there exist a need to provide alternative and suitable microphone arrangements in a hearing aid, which distinguishes the contribution from vibrations.